Chargino mass determination at a muon collider

We analyze the prospects at a muon collider for measuring chargino masses in the $\mu^+\mu^-\to \tilde{\chi}^+\tilde{\chi}^-$ process in the threshold region. We find that for the lighter chargino of a mass $100-200$ GeV, a measurement better than $50-300$ MeV should be possible with 50 fb$^{-1}$ integrated luminosity. The accuracy obtained here is better than with other techniques or at other facilities. The muon sneutrino mass, which enters through the $\tilde{\nu}_\mu$ exchange diagram, can also be simultaneously measured to a few GeV if it is not too heavy.